Method for forming road and ground constructions

ABSTRACT

A method for forming load-bearing road and ground constructions on a ground bed having a low load-bearing capacity includes assembling rigid support elements on the ground bed, forming a wearing layer above the support elements and disposing a light material having a bulk density lower than the ground bed below the support elements. The support elements are designed to be rigid so that point loads are distributed over the supporting surface of the ground bed. Before positioning the support elements, the ground bed is excavated to a predetermined depth such that the weight of the excavated ground mass in the area in which the support element is to be positioned corresponds essentially to the weight of the support element itself. By combining the heavier materials in the support elements and the light materials therebelow, the supporting structure is formed with a bulk density which does not exceed the average bulk density of the excavated ground mass so that the load exerted on the ground bed by the weight of the support structure and some of the dynamic loads absorbed by the support structure are compensated by the load reduction associated with the excavated ground mass. Any excess dynamic load exerted on the ground bed through the support structure is temporarily absorbed by the pore water pressure present in the ground bed.

FIELD OF THE INVENTION

This invention relates to a method for forming the foundation of groundand road constructions on beds having a low carrying capacity such asclay, peat, mud and water.

BACKGROUND OF THE INVENTION PRIOR ART

Road and ground constructions consist essentially of a wearing coursedisposed on the top, and, below it, a base course formed in varyingthicknesses from a well-defined sand or gravel material. In cases wherethe ground layers have a particularly low carrying or load-bearingcapacity, subbases can be added, these also being formed from a definedcomposition. A characteristic feature of conventional road and groundsurfacings is that the base courses can only tolerate small tensilestresses. The function of the base course is essentially one of loaddistribution or, in other words, increasing the surface area influencedby the point loads which are exerted on the wearing course to anacceptable level. The tensile stresses which are formed in the basecourse are dissipated as friction in the underlying earthen mass.

Conventional road surfacings are made up of base courses and wearingcourses whose bulk density is at least as great as that of theunderlying ground. Consequently, road surfacings having considerablydifferent bulk densities exist for various soil types. For example,well-graded, packed, sandy gravel has a bulk density of 1800-2000 kg/m³; clay, 1500-1600 kg/m³ ; and peat, 1000-1100 kg/m³.

Bitumen stabilization is used to increase the tensile strength of basecourses and, especially, the ability to withstand short-term loads.Various construction procedures including, for example, the use of fiberfabric mats, increase the tensile strength of both the base course andthe underlying earthen mass. Cement or lime stabilization of theunderlying ground, or the like, is primarily intended to increaserigidity. At the same time, the tensile strength also increases. Othermeasures for increasing the load-bearing capacity of the base coursesand for transferring tensile stresses include laying out horizontalpiles with end anchors or grillages of wood. Concrete, either plain orreinforced, is also used as a construction material. The concretegenerally constitutes a wearing course, but also contributes todistributing point loads along the underlying ground layers. By virtueof the reinforcement, the tensile strength of the concrete isconsiderably improved. Even if the density of the concrete is 2300-2400kg/m³, the reduced thickness required for the base course results in acorresponding reduction in the intrinsic load exerted by the entireconstruction. Plastic-molded concrete tends to shrink over time, therebycausing uncontrollable crack formations to occur. Therefore, concretesurfacings are generally provided with joints intended to function ascrack inhibitors. With such joints, the capacity of the wearing courseto tolerate tensile stresses caused by bending moment is reduced. Inorder to prevent extensive settling due to such joints, the base courseis typically chosen to be relatively thick, thereby increasing the loadexerted by the entire construction. The load can be reduced to someextent by producing the wearing course from light ballast concrete. Inthe United States, for example, light ballast concrete with a density aslow as 1600 kg/m³ has been used with good results. Concretes havinglower densities, however, have too little abrasion resistance and arequickly and easily worn down by traffic.

The load exerted on the underlying earthen masses can be reduced evenfurther in several ways. Materials with low bulk density, such as slag,haydite and cellular plastic, have been used to reduce the weight ofroad embankments. Piles driven into the ground may also be used totransfer the load from the roadway down to deeper-lying earthen layershaving a higher load-bearing capacity and rigidity than those lyingabove. The piles can be provided with pile helmets, or a reinforced,continuous concrete slab can be cast and supported by the piles. Thebase course and wearing courses are then formed above the slab. In thisarrangement, the load-bearing capacity of the underlying earthen layersis not utilized, the construction being comparable to the absorption ofall loads in the supports of a bridge. At the same time, loading anddrainage of the ground results in a packing effect in which the porositydecreases, as does the pore water pressure. For ground having a lowload-bearing capacity, the upper ground layers are often drained by asystem of pipes, and a preliminary load is applied by the laying ofsubbase and base courses. Vertical drainage is also performed in orderto shorten the consolidation time upon loading. In this way, anyextensive settling of the ground occurs before the wearing course isapplied. Such construction is typically provided with pipes for leadingsurface water away and for preventing a rise in ground water. In manycases where the ground is extremely inhomogeneous, the worst earthenmasses are removed before pre-loading is applied. The ability to detectthose areas which may result in particularly extensive settling isenhanced with pre-loading and simultaneous drainage.

Additional deformation in the form of so-called frost damage may alsooccur as the ground freezes. Such deformation may appear where frostprotection material is located under road and ground constructions. Thedamage occurs when the ground water is conveyed in capillary fashion infine-grained earth up to the freezing zone where an accumulation takesplace and ice crystals are formed. Freezing occurs more easily when thesurface construction is exposed, as with snow plowed roads havinginsulating snow banks along the sides thereof. The material in theroadway has little heat-insulating power so that the freezing isconcentrated in the areas under the roadway itself. In order to preventfrost damage, the frost-susceptible material must be removed and theground drained under the construction by pipe drainage. In order toimprove the heat-insulating power of the roadway, insulating materialssuch as haydite and slag can be used.

The ability of a soil type to absorb loads with subsequent deformationsdepends on the particle size and distribution, the degree of compactionand the pore water pressure in the intermediate space between the soilparticles. In loose soil types, such as clay and peat, the soilstructure itself can only bear a load for which the soil layer haspreviously reached an equilibrium, i.e., the pre-consolidation pressure.When the load increases beyond this pressure, the excess load isinitially absorbed by the pore water pressure in the soil layer. Thispressure changes with time, the change depending on the permeability ofthe soil, otherwise known as the dewatering rate. As a certain volume ofwater is squeezed out of the soil, a corresponding deformation orsettling in the ground layers occurs. This settling is irreversible.

The load-bearing capacity and rigidity of the earthen masses increasewith increasing depth as overlying layers become compacted and dewateredover time. Thus, a so-called consolidation takes place. If a certaincritical load is exceeded, the deformations increase quickly. Whendesigning constructions on cohesive soils, either the load must be belowthis critical load, or the load must be transferred via piles down tosoil layers having greater load-bearing capacity. Since ground layersare heterogeneous, the critical load tends to vary from place to place.

Since the surface of road and ground constructions lies above theadjacent ground to permit water runoff, loads exerted on theseconstructions are transferred to underlying ground layers. The porewater between the soil particles in the ground layers is drained off andremaining deformations appear. Settling in the ground layers is morepronounced with low pre-consolidation pressure. Such ground layersrequire, moreover, thicker base courses in order to increase the areainfluenced by the point loads exerted on the wear surfaces. This in turnresults in greater overall loading on the ground layers and, hence,increased settling.

SUMMARY OF THE INVENTION

The present invention is based upon achieving improved load distributionin road and ground constructions by introducing increased rigidity inthe upper part of such constructions. As a result, stresses anddeformations in the subsoil are reduced. Moreover, the pore waterpressure is used to absorb short-term loads having a concentrated loaddistribution, such as dynamic loads, which are exerted on theconstruction. The constructions are adapted and designed in such a waythat a completely compensated foundation is provided. By designing theconstructions to be relatively light, the foundation does not apply anyadditional load to the underlying soil layers than that which had beenapplied by the excavated surface soil and therefore does not change thepore water pressure in such underlying soil layers. In this connection,drainage of the ground layers as a result of overload is avoided, as isthe accompanying settling in the subsoil, particularly with respect toshort-term loads. Moreover, high ground water levels are no longer adisadvantage.

The supporting construction is formed from a composite of floatingbodies including a continuous beam grid whose rigidity and load-bearingcapacity are adapted to the properties of the ground, the magnitude ofthe load to be applied and the load-bearing capacity of the floatingbodies, each of the bodies resting on, but not anchored to, the groundbed. Point load stresses are balanced out and stress concentrations arereduced by means of the rigidity of the construction and by means ofdispersing the load via the floating bodies to the ground layers. Inaddition, the constructions are designed to be heat-insulating toprevent the underlying ground from freezing, thereby reducing frostdamage.

The road and ground constructions of the present invention are made upof prefabricated foundation elements for assembly on site. The elementsmay be chosen to accommodate either the full width of the constructionor parts thereof. The lower portion of the elements are formed fromcellular plastic or an equivalent material which, during casting, formsshaped recesses for the beam grid which is intended to form the upperpart of the element. More precisely, the construction of the presentinvention is accomplished by excavating earthen mass from the ground toform a ground bed and positioning a plurality of rigid foundationelements in edgewise fashion on the ground bed so that each one of theplurality of foundation elements is separated from an adjacent one ofthe plurality of foundation elements by a predetermined space. Theearthen mass is excavated to a predetermined depth so that the totalweight of the foundation element positioned on a portion of the groundbed is substantially equal to the total weight of the earthen massexcavated to form that portion of the ground bed. Each of the foundationelements includes a plurality of longitudinal and transverse beammembers arranged to form a beam grid, a low density material disposedbetween the beam members and a slab-like member disposed above the beamgrid. Although the beam members and the slab-like members are formedfrom a heavier material, the combination of these members with the lowdensity material is such that the bulk density of the foundation elementis no greater than the average bulk density of the excavated earthenmass.

After positioning, a material is deposited over the plurality offoundation elements wherein the deposited material fills thepredetermined spaces to join adjacent ones of the foundation elementsand wherein the deposited material forms a wearing layer above theplurality of foundation elements.

In one embodiment, the beam grid is formed from a reinforced lightballast concrete having a bulk density of between about 800 kg/m³ andabout 1400 kg/³.

In another embodiment, the wearing layer comprises a cast concrete layerhaving a thickness between about 20 mm and about 60 mm, and acompression strength between about 50 MPa and about 300 MPa, preferablybetween about 60 MPa and about 150 MPa. In more preferred embodiments,the cast concrete layer includes a reinforcing grid formed from amaterial selected from the group consisting of steel, glass, carbon andpolymers.

In still more preferred embodiments, the low density material comprisesa cellular plastic having a low water absorption. Preferably, thecellular plastic is provided in the form of solid blocks. Alternatively,the cellular plastic may be provided in a granular form. The cellularplastic preferably has a bulk density of between about 10 kg/m³ andabout 500 kg/m³ and more preferably between about 20 kg/m³ and about 100kg/m³.

In still more preferred embodiments, the pore water pressure extant inthe ground after construction of the road or ground construction issubstantially equal to the pre-excavation pore water pressure.

Preferred methods in accordance with the present invention provide roador ground constructions wherein the load exerted on the ground bed bythe weight of the foundation elements and at least some of the dynamicloads exerted on the foundation elements are compensated by the weightreduction associated with the excavated earthen mass. Any excess dynamicload exerted on the foundation elements, and in particular any dynamicload which exceeds the elastic deformation range for the elements, istemporarily absorbed by the pore water pressure present in the groundbed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a section of a road built inaccordance with the present invention;

FIG. 2 is a cross-sectional view taken along line I--I of FIG. 1;

FIG. 3 is a partial longitudinal cross-sectional view taken along lineII--II of FIG. 2;

FIG. 4 is a graphical representation of the vertical stresses in anearthen bed; and

FIG. 5 is a bottom plan view of the construction element shown in FIG.1, with the low density material removed to reveal the structure of thebeam grid.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The present invention is described hereinafter in connection with theelements used for forming the foundation of ground and roadconstructions. The concrete construction itself is designed as anassembly of longitudinal and transverse beams forming a beam grid 1which is covered, on its upper edge, by a continuous concrete slab 2.The beam grid and the slab are reinforced so that the necessary rigidityand load-bearing capacity are obtained in the finished construction. Thebeams of beam grid 1 and the slab 2 are formed from light ballastconcrete having a density of about 800-1400 kg/m³ and a compressionstrength between about 5-25 MPa. Particularly suitable concretes are 3Lconcrete and X concrete, both of which are structurally light ballastconcretes which have good frost resistance and provide good protectionagainst reinforcement corrosion and which are, in both these respects,fully comparable to high quality normal concrete. However, since theabrasion resistance of these types of concretes is low, they areunsuitable for use in wearing courses.

Before positioning the support elements, consisting of beam grid 1 andslab 2, for forming the foundation of the construction, the surfacelayers of the ground must be excavated. Such excavation is preferablycarried out where the weight of the excavated earthen mass willcorrespond to the magnitude of the load to be applied by theconstruction itself. The depth of the excavation is usually from severaldecimeters up to half a meter, if the height of the roadway is notdetermined by other considerations. The excavation can be carried out ina conventional manner without the need for support walls on the sidesthereof. The ground does not have to be strengthened or drained, butonly evened out with, for example, sand or gravel. Furthermore, basecourses or subbase courses do not have to be added, and measures forpreventing frost damage are also unnecessary.

When the support elements have been laid in place in the excavation bycranes or other lifting arrangements, the elements are locked in such away that they can function as a base for positioning of subsequentelements or as a base for supporting the lifting arrangement. The heightof the positioned elements should be adjustable as desired. Once set inplace, a 20-60 mm thick reinforced concrete layer 3 is cast on top ofthe elements. The concrete for forming layer 3 is chosen to be of thehigh-strength type having compression strengths preferably in the rangeof 50-300 MPa, and more preferably in the range of 60-150 MPa. Thisconcrete is used also for casting together the joints 4 between theelements. By reinforcing the cast layer with fiber nets of greatanchoring capacity, the construction can be designed without joints inthe top surface. Such fiber nets are typically formed from steel, glass,carbon or polymers. The shrinkage of the wearing layer leads to manyfine cracks which are of no importance for either the functioning or thestability of the construction. It is also possible to use fibers andfiber mats, both of steel and of glass or polymer material. Thehigh-strength concrete may be worked on the top surface to producesurface grooves which provide vehicles with enhanced gripping power. Thegroove design also enhances water runoff from the roadway. Damage to theroadway is easy to repair with the high-strength concrete.

The properties of the concrete used in the construction of the presentinvention are adapted so as to provide low intrinsic weight and, at thesame time, maximum load-bearing capacity and rigidity. The high-strengthconcrete used for the wearing course functions as a compression zonewithin the areas in which the bending moments are at their greatest. Thelight ballast concrete is then situated in the tension zone and affectsneither the moment capacity nor the rigidity. The high-strength concreteon the upper edge of the construction increases the punch resistancewhen this layer is compressed. The spaces between the beams of grid 1are filled with a cellular plastic 5 or similar low density materialwhich has a low water absorption, yet which has a sufficient rigidityand load-bearing capacity to absorb the deformations of thesuperstructure and disperse these to the underlying ground. Cellularplastics having a bulk density between about 10 kg/m³ and about 500kg/m³ are preferred; bulk densities between about 20 kg/m³ and 100 kg/m³are more preferred.

The load applied to the underlying ground can be made small incomparison to that applied by a conventional superstructure. In orderachieve the desired load-bearing capacity of the road bed, dimensioningof the superstructure is based upon load transfer of wheel pressurefrom, for example, asphalt layers to the underlying ground according totheories of elasticity. Since the stress is a function of the quotientE₁ /E₂, the greater the modulus of elasticity of an overlying layer(E₁), the less the stress and deformation of the underlying ground(having a modulus of elasticity of E₂).

FIG. 4 shows the vertical stress, in a two-layer bed of varying depth,immediately under the load as a function of the quotient between themoduli of elasticity of the layers. The area designated "A" correspondsto the stress in the upper layer and the area designated "B" correspondsto the stress in the lower layer, while "C" indicates the boundarysurface between the layers. The vertical stress is given along the xaxis and the thickness of the layers is given along the y axis. Thevertical stress in the upper layer of the two-layer bed can be read forvarious depths along the upper portion of each curve in area "A", whilethe vertical stress in the lower layer of the bed can be read forvarious depths along the lower portion of each curve in area "B".

The stress peaks from traffic load are dependent on irregularities inthe roadway. In cases of superimposed stresses or a long load period,plastic deformation and a reduced modulus of elasticity can also occurwhich, over and above the breakdown of the surface, impair the functionof the road bed. In conventional road constructions there are also localdifferences in the properties and thickness of the bed layers as aconsequence of differences in material and shortcomings in the layingtechnique. The asphalt layer is fatigued with time by dynamic loadswhich therefore accelerates the breakdown process. The dimensioningcriteria are, in principal, the same for the present invention. Breakageunder wheel load is not a dimensioning criterion in this constructionand therefore this type of pressure can, in principal, be increased. Byemploying a high-strength concrete layer in accordance with the presentinvention, the upper layer is formed with a high modulus of elasticitywhich does not change over time. Factors such as wearing, breakdown,handling and material inadequacies are of secondary importance, and theroad and the construction acquire a good service index, i.e., a high PSInumber.

Temperature fluctuations and gradients are considered when dimensioningthe construction of the present invention. The stresses which developfrom prevented deformation can be dissipated without the construction'sfunction being impaired.

Loads from the road or ground construction are thus transferred to theground, and the long-term loads exerted by the construction itself areessentially of the same magnitude as prevailed in the undisturbed earth.Point loads applied to the rigid superstructure are distributed suchthat the stress developed in the subsoil is below the critical value bya safe margin. In conventional road constructions the heavy intrinsicweight of the superstructure considerably reduces this safety margin.

Exceptional loads in the form of short-term loads which exceed thecritical load or pre-consolidation pressure are absorbed by the porewater pressure which prevails in the ground soil. This characteristic ofthe ground soil is unaffected by dynamic load since the earth has lowpermeability for water flow.

The lower parts of the constructions which consist primarily of cellularplastic 5 or the like, are heat insulating, thereby eliminating damagedue to frost. Further advantages resulting from the present inventionare that the ground does not have to be drained and ditch drainage isavoided. The road follows the ground contour, and a leveling out onaccount of local differences is achieved.

What is claimed is:
 1. A method for forming load-bearing road and groundconstructions on ground having a low load-bearing capacity comprisingthe steps of:excavating earthen mass from said ground to form a groundbed, positioning a plurality of rigid foundation elements having a firstaverage bulk density in edgewise fashion on said ground bed, each ofsaid foundation elements positioned on a corresponding portion of saidground bed, each one of said plurality of foundation elements separatedfrom an adjacent one of said plurality of foundation elements by apredetermined space and including a plurality of longitudinal andtransverse beam members arranged to form a beam grid, a low bulk densitymaterial disposed between said beam members and a slab-like memberdisposed above said beam grid, and depositing a material having a secondaverage bulk density greater than said first average bulk density oversaid plurality of positioned foundation elements wherein said depositedmaterial fills said predetermined spaces to join adjacent ones of saidfoundation elements and wherein said deposited material forms a wearinglayer above said plurality of foundation elements, wherein thecombination of said first and second bulk densities provides a totalweight substantially equal to the total weight of a portion of saidearthen mass excavated to form said corresponding portion of said groundbed.
 2. A method as claimed in claim 1, wherein said beam grid is formedfrom a reinforced light ballast concrete having a bulk density ofbetween about 800 kg/m³ and about 1400 kg/m³.
 3. A method as claimed inclaim 1, wherein said wearing layer comprises a cast concrete layerhaving a thickness between about 20 mm and about 60 mm.
 4. A method asclaimed in claim 3, wherein said cast concrete layer has a compressionstrength between about 50 MPa and about 300 MPa.
 5. A method as claimedin claim 4, wherein said cast concrete layer has a compression strengthbetween about 60 MPa and about 150 MPa.
 6. A method as claimed in claim3, wherein said cast concrete layer includes a reinforcing grid.
 7. Amethod as claimed in claim 6, wherein said reinforcing grid is formedfrom a material selected from the group consisting of steel, glass,carbon and polymers.
 8. A method as claimed in claim 1, wherein said lowbulk density material comprises a cellular plastic having low waterabsorption.
 9. A method as claimed in claim 8, wherein said cellularplastic is provided in the form of solid blocks.
 10. A method as claimedin claim 8, wherein said cellular plastic is provided in a granularform.
 11. A method as claimed in claim 8, wherein said cellular plastichas a bulk density of between about 10 kg/m³ and about 500 kg/m³.
 12. Amethod as claimed in claim 11, wherein said cellular plastic has a bulkdensity of between about 20 kg/m³ and about 100 kg/m³.
 13. A method asclaimed in claim 1, wherein said ground has a pre-excavation pore waterpressure and a post-construction pore water pressure, saidpost-construction pore water pressure at a specific depth and locationbeing substantially equal to said pre-excavation pore water pressure atsaid specific depth and location.